A problem that is encountered in the use of conventional IR radiation detectors is a reduction in photocarrier lifetime due to the presence of impurities within the semiconductor material that comprises the active, radiation absorbing region. Of particular interest herein are radiation detectors that are constructed of Group II-VI semiconductor material, such as the material mercury-cadmium-telluride (Hg.sub.1-x Cd.sub.x Te), wherein x is a non-zero number less than one. The value of x is selected to achieve a desired bandgap of the Group II-VI material, which in turn determines a range of wavelengths that are absorbed by the material.
The impurities function as recombination sites wherein photogenerated charge carriers (electrons and holes) are effectively removed from the semiconductor material. This results in a reduction-in both responsivity and detectivity.
It is known in the art to provide a wider bandgap semiconductor material at a surface of semiconductor material radiation absorbing layer, the wider bandgap surface material functioning as a passivation layer to reduce surface noise states. For example, reference is made to the following two commonly assigned U.S. Patents for showing photovoltaic IR radiation detectors that include a wider bandgap layer that overlies a radiation absorbing HgCdTe layer: U.S. Pat. No. 4,956,304, "Buried Junction Infrared Photodetector Process", to C. A. Cockrum, J. B. Barton, and E. F. Schulte; and U.S. Pat. No. 4,961,098, "Heterojunction Photodiode Array", to J. P. Rosbeck and C. A. Cockrum. This latter U.S. Patent also shows the use of a compositionally graded radiation absorbing layer and a compositionally graded, wider bandgap cap layer.
Commonly assigned U.S. Pat. No. 5,079,610, "Structure and Method of Fabricating a Trapping Mode", to P. R. Norton shows the use of a wider bandgap p-type layer for trapping minority carrier holes. A p-n junction is employed in this device for separating carriers. Commonly assigned U.S. Pat. No. 4,914,495, "Photodetector with P Layer Covered by N Layer", to P. R. Norton, M. Moroz, and C. S. Talley shows the use of an n-type layer that substantially completely overlies a p-type layer. The p-type layer is employed for trapping minority carriers holes.
Commonly assigned U.S. Pat. No. 4,885,619, "HgCdTe MIS Device Having a CdTe Heterojunction", to K. Kosai describes a metal-insulator-semiconductor (MIS) device that includes a HgCdTe substrate having a CdTe passivation layer.
Also of interest are U.S. Pat. No. 4,357,620 for showing a Liquid Phase Epitaxy (LPE) process that grows a HgCdTe epilayer on a CdTe substrate, and a CdTe epilayer upon the HgCdTe epilayer; U.S. Pat. No. 4,376,659 for showing a narrow-gap semiconductor layer that is deposited upon a wider-gap semiconductor layer; and U.K. Patent Application No. GB 2 100 927 for showing a photodiode having a HgCdTe substrate and a CdTe layer, wherein heating causes diffusion between the HgCdTe and CdTe to provide a graded heterostructure.
What is not provided in radiation detectors of the prior art, and what are thus objects of this invention to provide, are (a) a radiation detector having a "denuded zone" of semiconductor material of high purity; (b) a radiation detector having a compositionally graded radiation absorbing region that generates an electric field for injecting minority photocarriers into the denuded zone; and (c) a radiation detector having a graded composition passivation layer to both getter impurities and reduce surface recombination effects.